[Spreadsheet] Fix input in expression editor (#19934)

* [Spreadsheet] Fix input in expression editor

Closes #19804

* [pre-commit.ci] auto fixes from pre-commit.com hooks

for more information, see https://pre-commit.ci

---------

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

src/Gui/ExpressionCompleter.cpp

@@ -956,6 +956,11 @@ void ExpressionLineEdit::setExactMatch(bool enabled)
     }
 }
 
+ExpressionCompleter *ExpressionLineEdit::getCompleter(void)
+{
+    return this->completer;
+}
+
 bool ExpressionLineEdit::completerActive() const
 {
     return completer && completer->popup() && completer->popup()->isVisible();